Recently, a new class of thermoelastic sulfonated polymers has been described in a number of U.S. patents. These sulfonated polymers are derived from polymeric materials having olefinic unsaturation, especially elastomeric polymers such as Butyl and EPDM rubbers. U.S. Pat. No. 3,642,728, herein incorporated by reference, clearly teaches a method of selective sulfonation of olefinic unsaturation sites of an elastomeric polymer to form an acid form of a sulfonated elastomeric polymer. The olefinic sites of the elastomeric polymer are sulfonated by means of a complex of a sulfur trioxide donor and a Lewis base. The SO.sub.3 H groups of the sulfonated elastomer can be readily neutralized with a basic material to form an ionically cross-linked elastomer having substantially improved physical properties over an unsulfonated elastomer at room temperature. However, these ionically cross-linked elastomers, may be processed like a conventional thermoplastic at elevated temperatures under a shear force in the presence of selected preferential plasticizers which dissipate the ionic associations at the elevated temperatures thereby creating a reprocessable elastomer.
The basic materials used as neutralizing agents are selected from organic amines or basic materials selected from Groups I, II, III, IV, V, VI-B, VII-B and VIII and mixtures thereof of the Periodic Table of Elements. Although these sulfonated elastomeric polymers prepared by the process of this patent are readily useable in a certain number of limited applications, they are not as readily adaptable for the manufacture of an extrudable and injection moldable elastomeric article as are the improved compositions of the present invention, wherein both improved compression set and rheological properties are realized.
U.S. Pat. No. 3,836,511, herein incorporated by reference, teaches an improved process for the sulfonation of the olefinic sites of the elastomeric polymer, wherein the improved sulfonating agent is selected from acetyl sulfate, propionyl sulfate and butyryl sulfate. The neutralizing agents employed to neutralize the acid form of the sulfonated elastomeric polymers are organic amines. The resultant ionically cross-linked sulfonated elastomers prepared by this process do not exhibit both the improved compression set and rheological properties of the compositions of the present invention.
U.S. Pat. No. 3,870,841, herein incorporated by reference, teaches a method of plasticization of the polymeric backbone of a neutralized sulfonated plastic polymer by means of a polymer chain plasticizer which is a liquid compound having a boiling point of at least about 120.degree. F. The polymer chain plasticizer is selected from a dialkyl phthalate, a process oil or an organic acid ester. Additionally, a domain plasticizer can be incorporated into the composition, wherein the domain plasticizer reversibly disrupts the association of the sulfonated groups at a temperature of forming. The compositions formed by this process are not suitable for the manufacture of high performance elastomeric articles formed by extrusion or injection molding process as are the compositions of the present invention.
U.S. Pat. No. 3,847,854, herein incorporated by reference, teaches a method of improving the processability of neutralized sulfonated elastomeric polymers by the addition of a preferential plasticizer which has at least one functional constituent which exhibits a bond moment whose absolute value is at least 0.6 Debyes, and must be a liquid at the desired processing temperature of the neutralized sulfonated elastomeric polymer. Again, the compositions of the present invention are more adaptable for use in the manufacture of high performance elastomeric articles.
Products resulting from the aforementioned methods for obtaining neutralized sulfonated elastomeric compositions, possess either unsuitable rheological properties or inferior compression set properties for the applications envisioned in the present invention.
For example, the physical properties of the resultant sulfonated elastomeric products of these aforementioned patents are unsuitable for major applications of automobile bumper parts, gaskets, weather stripping and various appliance parts.
U.S. Pat. No. 3,974,240 and No. 3,974,241 of the present inventors, filed on Nov. 18, 1974, describe the blending of a crystalline polyolefinic material with a neutralized sulfonated elastomeric polymer in an attempt to improve both the rheological and physical properties of the elastomeric polymer. The selection of the use of the crystalline polyolefinic material to improve both the stiffness as well as improving the melt viscosity of the composition was based in part upon the limitation of the use of fillers such as carbon black, clays, calcium carbonate or silicates as a single additive to the elastomeric polymer. Although fillers in combination with an elastomeric polymer increase the hardness of the composition, these fillers deteriorate the melt viscosity of the resultant composition. These materials are more adaptable for stiff elastomeric articles such as rubberized chair tips or wheels whereas the compositions of the present invention are more adapted for flexible elastomeric articles such as gaskets.
U.S. Pat. Nos. 4,220,573; 4,313,867; 4,118,360; 4,118,353; 4,166,751; 4,134,870; 4,151,137; 4,157,992; and 4,169,820 all teach the concept of using sulfonated polymers for the formation of elastomeric articles; however, none of these U.S. patents teach compositions with the improved compression set properties as exhibited by the compositions of the instant invention.
Sulfonated elastomeric polymers are somewhat defensive regarding their compression set characteristics when compared with other thermoplastic elastomers, e.g., block copolymers such as polyurethanes and very defensive when compared with vulcanized elastomers. In the past, various efforts have been made to improve their poor compression set characteristics. However, so far all the attempts to achieve the desired level of compression set by a process which can be considered a practical one have been unsuccessful.
Broadly speaking, compression set is a measure of the flexibility and elasticity of the material. Most often to measure the compression set of an elastomer, standard ASTM test (D-395) is used. The test is designed in such a way that it measures the material's ability to retain its elastic properties after a prolonged exposure to compressive stresses.
The present invention teaches that sulfonated EPDM samples varying in the type of cation when blended in small quantities with their precursor EPDM polymers or EP copolymers with similar structures yield compositions which have significantly improved compression set properties.
The unique and novel improved compositions of the present invention overcome the deficiencies of the aforementioned U.S. patents and applications from both a rheological and compression set properties aspect. The blend compositions of the present invention solve the problem of having a material which has both desirable rheological and compression set properties for the manufacture of an elastomeric article such as a gasket wherein the extrudate of the resultant compositions do not exhibit melt fracture during extrusion processing as is the case in some of the aforementioned patents.